Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into three general categories, namely, fuel cells utilizing compressed hydrogen (H2) as fuel; proton exchange membrane (PEM) fuel cells that use methanol (CH3OH), sodium borohydride (NaBH4), hydrocarbons (such as butane) or other fuels reformed into hydrogen fuel; and PEM fuel cells that use methanol (CH3OH) fuel directly (“direct methanol fuel cells” or DMFC). Compressed hydrogen is generally kept under high pressure and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices.
DMFC for relatively larger applications typically comprises a fan or compressor to supply an oxidant, typically air or oxygen, to the cathode electrode, a pump to supply a water/methanol mixture to the anode electrode and a membrane electrode assembly (MEA). The MEA typically includes a cathode, a PEM and an anode. During operation, the water/methanol liquid fuel mixture is supplied directly to the anode and the oxidant is supplied to the cathode. The chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Reaction at the cathode:O2+4H++4e−→2H2O
The overall fuel cell reaction:CH3OH+1.5 O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode through the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. The external circuit may be any useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others. DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion™. available from DuPont, which is a perfluorinated material having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
The cell reaction for a sodium borohydride reformer fuel cell is as follows:NaBH4 (aqueous)+2H2O→(heat or catalyst)→4(H2)+(NaBO2)(aqueous)H2→2H++2e− (at the anode)2(2H++2e−)+O2→2H2O (at the cathode)
Suitable catalysts include platinum and ruthenium, among other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. Sodium borohydride fuel cell is discussed in U.S. published patent application no. 2003/0082427, which is incorporated herein by reference.
One of the important features for fuel cell applications is fuel storage. Another important feature is the regulation of the transport of fuel out of the fuel cartridge to the MEA. To be commercially useful, fuel cells such as DMFC systems should have the capability of storing sufficient fuel to satisfy a consumer's normal usage. For example, for mobile or cell phones, for notebook computers, and for personal digital assistants (PDAs), fuel cells need to power these devices for at least as long as the current batteries, and preferably much longer. Additionally, fuel cells should have easily replaceable or refillable fuel supplies to minimize or obviate the need for lengthy recharges required by the current rechargeable batteries.
Suitable fuel supplies can be either disposable cartridges or refillable cartridges. For the disposable cartridges, the consumer must carry a sufficient supply of spare fuel cartridges for each electronic device. Absent a correct spare cartridge, fuel cannot be supplied to the fuel cell. In addition, once these fuel cartridges are empty, they cannot be refilled and are simply discarded.
Refillable fuel cartridges alleviate having to discard empty cartridges. However, it is desirable to be able to refill these fuel cartridges in a simple and efficient manner, and to be able to refill the empty cartridges in-situ without having to remove them from the electronic device. Additionally, it is desirable to provide a system for transferring fuel from the fuel cartridge in one electronic device to the fuel cartridge or to an internal fuel chamber in a second electronic device. Therefore, fuel can be shared among various electronic devices. Suitable fuel filling and transfer systems would be arranged to handle a wide variety of fuel cartridges.